Reversible squirrel-cage motor having low plugging torque



Aug. 9, 1932. J ,JQHNs 1,870,777

REVERSIBLE SQUIRREL CAGE MOTOR HAVING LOW FLUGGING TORQUE Filed Oct. 22. 1929 INVENTOR fianc/s J Jo/ms.

\ ATTORNEY Patented Aug. 9, 1932 UNITED STATES PATENT OFFICE FRANCIS J. JOHNS, F WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA REVERSIBLE SQUIRREL-CAGE MOTOR HAVING LOW PLUGGING TORQUE Application filed October 22, 1929. Serial No. 401,575.

My invention relates to polyphase, ironcore, squirrel-cage motors which are designed for use on large washing machines of the type requiring the motor to be reversed, by plugging, approximately every ten seconds. A-motor of the type specified must have a low plugging torque in order to eliminate jerking of the machine and strain on the gears or other drive mechanism; it must be quiet during the plugging operation; it must have a reasonably small slip at full load; and it should have certain other characteristics which will be pointed out. 7 A My invention relates to a design of the slot arrangements and other features of a motor of the character mentionedabove to produce, for the first time, the characteristics just enumerated. V

In the drawing, Fig. 1 is a full-scale, frag mentary, transverse sectional View of the rotor ofa three-phase, -horsepower, 6-pole, (SO-cycle, 11(JO-R. P. M. motor embodying my invention, and Fig. 2 is an enlarged view of a rotor slot with the conductor therein shown in section.

In the drawing, the steel rotor punchings 2 of my motor are provided with a plurality of shallow openings or slots 5 in which are disposed the squirrel-cage bars 4 preferably formed of hard-drawn copper and rectangular in cross-section. The bars a are buried intherotor to provide a bridge 5 over each of the slots or openings 3, and each bridge has a radial slit 6 which is preferably as narrow,,0r.nearly ,asnarro'w, as it is economical- 1y. practical to make it without unduly curtailing the life of the punching die and which extends upwardly from the top of the slot 3 to within a few hundredths of an inch of the outer periphery of the punching 2. At present, I am making the slits between 31 and mils in width, in the diiferent motors embodying my invention.

After the rotor is finished and mounted on its shaft, with the squirrel-cage bars 4 in place, its periphery is turned down, as indicated by the broken line 7 in the drawing, in order to make the periphery smooth and to adjust the air-gap to the desired depth, which may be something like two hundredths of an inch on each side of the rotor. This turning operation reduces the thickness of the solid iron bridge-portion 8 above the slit 6 to something of the order of 10 thousandths of an inch, which is sufiicient to prevent the cutting tool from catching on the slit and small enough to so limit the degree of saturation of the solid iron bridge-portion 8 as to avoid any serious harmful effect. The metal bridge 8 is figured in my motor calculations as having a slot factor of .5.

The attainment of desirable reversing motor characteristics, for the type of service hereinabove mentioned, has not been easy, and the-steps which I have taken in its accomplishment constitute departures from all previous designs. Thus, while I shall attempt to point out, as well as possible, in the followingexplanations, my reasons for introducing certain design-limitations, or my present theory as to the significance of certain designslimitations which may have been more the result of my own personal experience, rather than inductive reasoning, I do not Wish to be limited to the theory hereinafter stated, and I desire it to be understood that these explanations constitute merely my own present views and theories of the matter.

The plugging torque is perhaps the most significant item of my reversible motor. When I refer to plugging current and torque, I mean the current and torque which are obtained by reversing the terminals of one phase of the three-phase motor without any time-interval at the reversal and without any resistance inserted in the primary phases.

According to some systems of control, there may be a time interval of, say, not more than three seconds, and resistance may be included inthe reversed primary circuits to be cut out in steps by suitable control mechanism. These control systems have nothing to do with my present invention. I prefer to indicate the nature of my motor, however, by referring to its behaviour on plugging, by which I mean that no time interval or primary resistance is utilized.

CID

The plugging torque should be less than V the starting torque and should preferably be between .7 and .85 of the starting torque,

whereas, in all squirrel-cage motors of previous design, the plugging torque has been of the order 01: at least one and three-quarters to two times the starting torque and, in double-deck squirrel-cage motors, the plugging torque has been three to four times the starting torque. In order to get the pluggmg torque down to the requisite value, I believe that it is necessary to strictly limit the depth of the squirrel-cage bars which, in my design, are betweenfl20 and .30 inches, because the eddy-current action tends to crowd the current into the tops of the bars, thus increasing the plugging torque, which is very strong, even with ordinary bar-depths of around a half inch, at the double-frequency secondary Conditions existing at the instant of plugging. At the same time, the plugging current and the starting current must be made reasonably small, in order to reduce the power drawn by the motor, and the disturbance on the line, as a result of its frequent reversals. My plugging current is something of the order of more than the starting current, as

a ainst lu 'in currents of more than the starting currents in previous designs. My starting current is also lower than previous designs, ranging from 250 to 320% of the full-load current.

.- To'reduce the starting current, it has been 3 starting current is almost all'reactive, it is obviously reduced by this procedure, which is standard practice. In previous washingmachine motors for the class of service for.

which my present motor is designed, thegap flux density has been about 16,000 to 23,000

lines per square inch, as compared with a gap flux density, in the standard or general-purpose motor of from 85,000 to 42,000 lines per square inch, running as high as 45,000 lines per square inch in ordinary high-startingtorque motors.

Inmy present motor, I utilize only a slightly smaller'fiux density than in the standard general-purpose motor, going down to an extreme lower limit of 26,000 or 27,000 lines per I reduce my starting current by means ofthe slot'reactance of my deep-slit bridge 5 which, in my motor, has aslot-iactor (or ratio of the depth to the breadth of the slit) of from 2 to 8,.to which is to be added, in each-case, forthe ratio of the bar-depth to three times the bar-width) is from .25 to .85 in my new motor. The ratio of the slot-factor of the bridge to the slot-factor of the bar, in my new motor, is very high, in order to introduce a rotor reactance which will properly limit the starting current, ranging from values of 6 to 24 in my extreme design limits.

In general, I prefer to push the rotor-tooth density up to close to the highest practicable limits, which are around 100,000"Iines"per square inch or more, or from about 80,000 to 120,000 lines per square inch; whereas, in previous designs of reversible washing-machine motors, this rotor-tooth density has been smaller than 80,000 lines per square inch, with the result that the depth of the bars had to be made larger in order to get the requisite horsepower andother characteristics, which, in turn, meanta-very apprecia-- ble eddy-current eifect on plugging, resulting in relatively large plugging currents and still larger plugging torques. i The item of quiet operation is one about which very little is known. It can onlybe said that experience has shown that certain combinations of the number of rotor-slots with certain numbers of stator slots in-certain sizes of motors have been very good-and others very bad, from this standpoint. If

any generalization can be made, I believe that the smaller slot-numbers on therotor introduce the greater danger of noisy operation.

WVhile I have described my invention in a preferred form of embodiment,-I-do not desire to be-limited thereto, 'exceptcas'indicated in the appended claims, -when-read in the light of the prior art.

I claim as my invention:

l. A p'olyphase iron-core squirrel-cage motor having low plugging torque, characterized by having copper squirrel-cage bars and a slitted bridge over each bar; the

slot-factor of the bridge being approximately between 2% and 8 the slot-factor of the bar being approximately between .25 and .85;-the ratio of said slot-factors being ap proximately betweenfi and 24; thegap-"flux density being approximately between 27,000

and 86,000 lines :per square'inch; the rotor tooth density being approximatelybetween 80,000 and 120,000 lines per square inch and the bar-depth being approximately between .20 and .80 inch.

2. A polyphase iron-core squirrel-cage motor having low plugging torque, char acterized by having copper squirrel-cage bars and a slitted bridge over each bar; the gap flux, density being greater than about 26,000- lines per square inch; the bar-depth being less than about .30 inch; and the ratio of the slot-factor of the bridge to the slot-factor of the bar being greater than about 6.

3. A .polyphase iron-core squirreLcage motor having low plugging torque, characterized by having copper squirrel-cage bars and a slitted bridge over each bar; the squirrel-cage bars being substantially rec tangular and having a slot-factor approximately between .25 and .85; the bridge hav ing a slot-factor at least about 6 times that of the bar; and the bar-depth being less than about .30 inch.

4. A polyphase iron-core squirrel-cage motor having lOW plugging torque, characterized by having copper squirrel-cage bars and a slitted bridge over each bar; the bardepth being less than about .30 inch; the slot factor of the bridge being greater than about 2 and the ratio of the slot factor of the bridge to the slot factor of the bar being greater than about 6.

5. The invention, as defined in claim 1,

characterized by the fact that the slits in the bridges are closed at the air-gap periphery by an iron core-portion having a depth of the order of 10 mils.

6. The invention, as defined in claim 4:, characterized by the fact that the slits in the bridges are closed at the air-gap periphery by an iron core-portion having a depth of the order of 10 mils.

In testimony whereof I have hereunto subscribed my name this 18th day of October,

FRANCIS J. JOHNS. 

